Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops onto an image receiving surface from a plurality of inkjets, which are arranged in one printhead or an array of printheads. For example, in some embodiments an image receiving member with an image receiving surface moves relative to the printheads and a controller operates individual inkjets in the printheads to eject ink drops at appropriate times based on image data to form a printed image in the image receiving surface. In some embodiments, the image receiving surface is the surface of a drum or endless belt that transfers a latent ink image received from the inkjets in the printhead array to a paper substrate or other suitable print medium. In other embodiments, the inkjets in the printheads eject drops of ink directly onto the surface of the print medium.
During operation of the printer, some inkjets may become inoperable and fail to eject ink drops in a reliable manner. An inoperable inkjet produces a noticeable streak extending along a length of the printed image in locations where the inkjet would have printed ink drops if the inkjet was operating normally. Prior art inkjet printers employ compensation techniques to reduce the visibility of these streaks. For example, FIG. 8 depicts a sample of contone image that compensates for two inoperable inkjets. The image data includes streaks that correspond to the locations of two inoperable inkjets in groups 804 and 808. FIG. 8 depicts a total of six streaks because in the embodiment of FIG. 8 the printer generates an image using three passes of the inkjets, including the inoperable inkjets, with a cross-process direction translation of the printheads between passes. Each of the streaks is arranged along a raster of the image data corresponding to a location of the inoperable inkjets. The compensation process adjusts the contone image data for a predetermined number of columns of image data on either side of the streaks, which enables neighboring inkjets in the printer to compensate for the inoperable inkjets and produce printed images that reduce or eliminate the visual impact of the streaks from the inoperable inkjets.
One deficiency of the image data compensation technique depicted in FIG. 8 is that the adjusted contone image data in the regions around the inoperable inkjet that compensates for the local streak defect also changes the distribution of ink drops that form the printed image after the printer performs a halftone conversion operation using the adjusted contone image data. Using different combinations of ink drops to form similar colors on different parts of a single printed image has the potential to produce irregularities in the perceived colors in different regions of a single image. Consequently, improvements to the operation of inkjet printers that compensate for inoperable inkjets would be beneficial.